def is_prime(n):
    if n<=1:
        return False
    for i in range(2,int(n**0.5)+1):
        if n%i==0:
            return False
    return True

def max_prime(number):
    if len(str(number))!= 4or number<1000:
        return 0

    digits=str(number)
    max_prime_num=0

    for i in range(4):
        for j in range(i+1,5):
            num=int(digits[i:j])
            if is_prime(num) and num>max_prime_num:
                max_prime_num = num

        return max_prime_num

number=int(input())
a=max_prime(number)
print(a)
